1. Field of the Invention
The present invention relates to a static type reactive power generator for adjusting the reactive power of an alternating current system.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a prior art static var generator using self-commutated inverter (hereinafter denoted by SVG) of this kind shown in, for example, "MITSUBISHI DENKI GIHO", Vol. 56, No. 6, 1982, pp. 47-52. In FIG. 1, reference numeral 1 designates a three-phase alternating current system (power system), and reference numeral 2 designates a reactor. Reference numeral 3 designates a multiple transformer, and reference numeral 4 designates pulse width modulation (hereinafter denoted by PWM) controlling system single-phase inverter connected in multistage. Reference numeral 5 designates a power capacitor as a d.c. power source.
Reference numeral 6 designates a potential transformer (hereinafter denoted by PT) for detecting each phase system voltage of the alternating current system 1. Reference numeral 7 designates a current transformer (hereinafter denoted by CT) for detecting each phase system current or current flowing in each phase reactor 2. Reference numeral 8 designates a synchronism detection circuit for detecting a phase reference .phi..sub.O for the PWM and a phase angle .theta..sub.O. Reference numeral 9 designates a d.c. voltage detector for detecting the d.c. voltage of the power capacitor 5 and outputting it as a d.c. voltage feedback signal Vd.sup.-. Reference numeral 10 designates a current detector as a current detecting means for outputting system phase current signals I.sub.U.sup.-, I.sub.V.sup.- and I.sub.W.sup.- detected by the CT 7 and a reactive current feedback signal deduced by executing a d-q transformation from these phase current signals and the above mentioned phase angle .theta..sub.O. Reference numeral 11 designates a voltage detector as a voltage detecting means for outputting an active voltage feedback signal Vp- deduced by executing the d-q transformation from each phase system voltage signal detected by the PT 6 and the phase angle .theta..sub.O.
Reference numeral 12 designates a protecting circuit for detecting abnormal phenomena produced by grounding accidents of the alternating current system from the d.c. voltage feedback signal Vd.sup.-, the system phase current signals I.sub.U.sup.-, I.sub.V.sup.- and I.sub.W.sup.- and the active voltage feedback signal Vp.sup.-, and the protecting circuit 12 generates a gate block (hereinafter denoted by GB) signal. Reference numeral 13 designates a PWM circuit for controlling gate signals of the signal-phase inverter 4. Reference numeral 14 designates a signal adder for operating the deviation between the d.c. voltage feedback signal Vd.sup.- and a d.c. voltage reference signal Vd*. Reference numeral 15 designates a d.c. voltage controller generating a phase difference reference signal .DELTA..phi. for d.c. voltage control from the output signal of the signal adder 14, and the d.c. voltage controller 15 is composed of proportional integrating elements.
Reference numeral 16 designates a signal adder for operating the deviation between the active voltage feedback signal Vp.sup.- and an active voltage reference signal Vp*. Reference numeral 17 designates a system voltage controller generating a reactive current reference signal I.sub.Q *for system voltage controlling from the output signal of the signal adder 16, and the system voltage controller 17 is composed of proportional integrating elements and the like. Reference numeral 18 designates a signal adder for operating the deviation between the reactive current feedback signal I.sub.Q.sup.- and the reactive current reference signal I.sub.Q *. Reference numeral 19 designates a reactive current controller for generating a conduction angle reference signal .theta.* for the reactive current control of the output signal from the signal adder 18, and the reactive current controller 19 is composed of proportional elements. Reference numeral 20 designates a reactive current reference zero controlling circuit for controlling the reactive current reference signal I.sub.Q * to zero over a prescribed period of time on the signal from the protecting circuit 12.
Next, the operation of the prior art SVG will be described. At first, the basic principle of the SVG will be described.
If the magnitude, frequency and phase of the output voltage of the SVG (the output voltage is the synthesized one from the output voltages of each single-phase inverter 4 by the multiple transformer 3) are synchronized with those of the power system voltage, the inflow current from the alternating current system (i.e. power system) 1 to the SVG is zero. However, if the output voltage of the SVG is controlled to be higher than the system voltage, an advanced phase current is flowed into the SVG. In opposition, if the output voltage of the SVG is made to be lower than the system voltage, a lagging current is flowed into the SVG.
The controlling of the reactive power can be done by controlling the output voltage of the SVG. And, there are two controlling methods for controlling the output voltage in general, one of them is the PAM method where the output voltage of an inverter is controlled by adjusting variably the d.c. voltage of a capacitor under the condition that the conduction angle .theta. of the inverter is constant, and the other of them is the PWM method where the output voltage of an inverter is controlled by adjusting variably the conduction angle .theta. of the inverter under the condition that the d.c. voltage of a capacitor is constant. FIG. 1 designates the latter PWM method. Besides, the d.c. voltage is controlled by the phase difference between those of the system voltage and the output voltage of the SVG, and the output voltage of the SVG is controlled by the conduction angle .theta. of the inverter.
Now, the output voltage of the inverter is represented with the following expression. ##EQU1## .theta.: conduction angle of inverter Ed: d.c. voltage
V.sub.OI : fundamental effective value of inverter output voltage
Besides, the d.c. voltage is controlled by the phase difference reference signal .DELTA..phi. calculated from the constant d.c. voltage reference signal Vd and the d.c. voltage feedback signal Vd.sup.- by the d.c. voltage controller 15.
FIG. 2-FIG. 4 simply shows the operation of the basic principle described above using a single-phase circuit. That is, in FIG. 2, the single-phase inverter 4 is composed of gate turn-off thyristors (hereinafter denoted by GTO thyristors) 41 connected in a single phase bridge connection to each other and diodes 42 connected to the GTO thyristors 41 in reversely parallel connections, and the output voltage V.sub.I of the inverter 4 is inputted to the primary winding side of a single-phase transformer 3a, and further the secondary winding side of the transformer 3a is connected to the alternating system 1 having a voltage V.sub.S.
FIG. 3a-e show the output voltage and current waveforms of the single-phase inverter 4 and the conduction states of each arm device T.sub.A, T.sub.B, T.sub.C and T.sub.D, respectively , when a lagging current is outputted by making the voltage V.sub.S and the output voltage V.sub.I to be V.sub.S &gt;V.sub.I.
And, FIG. 4a-e show the output voltage and current waveforms of the single-phase inverter 4 and the conduction states of each arm device T.sub.A, T.sub.B, T.sub.C and T.sub.D, respectively when an advanced phase current is outputted by making the voltage V.sub.S and the output voltage V.sub.I to be V.sub.S &lt;V.sub.I.
In the SVG shown in FIG. 1, the system voltage controller 17 calculates the reactive current reference signal I.sub.Q * on the active voltage feedback signal Vp.sup.- detected by the voltage detector 11 and the active voltage reference signal Vp*. Then, the reactive current controller 19 calculates the conduction angle reference signal .theta.* on the reactive current reference signal I.sub.Q * and the reactive current feedback signal I.sub.Q.sup.- detected by the current detector 10, and the reactive current controller 19 sends the conduction angle reference signal .theta.* to the PWM circuit 13. Since the phase reference .phi..sub.0 from the synchronism detection circuit 8 and the phase difference reference signal .DELTA..phi. from the d.c. voltage controller 15 are inputted separately to the PWM circuit 13, the PWM circuit 13 decides the gate pulses of each single-phase inverter 4 on these signals and sends the gate pulses to each single-phase inverter 4. The multiple transformer 3 synthesizes the output voltages of each single-phase inverter 4 and generates the output voltage V.sub.I as that of the SVG. The reactive power is generated from the difference voltage between this output voltage V.sub.I and the system voltage VS through the reactor 2, as described about the above mentioned principle.
Next, the operation in case of the accident of the alternating system especially aimed in the present invention will be described in FIG. 5(a)-FIG. 5(e). FIG. 5(a)-FIG. 5(e) show the operation waveforms of each signal in case of the occurrence of a one-line or a two-line grounding accident in the alternating system 1. FIG. 5(a) shows the waveform of the active voltage feedback signal Vp.sup.-. The oscillatory waveform of the second harmonic (2f ) component of the frequency (f) of the alternating system 1 appears during the period of the occurrence of the grounding accident, as will be described later.
FIG. 5(b) shows the d.c. voltage feedback signal Vd.sup.-. It falls greatly at the occurrence of the grounding accident, but it recovers immediately because the protecting circuit 12 operates to execute the gate block (GB) of the single-phase inverter 4 (refer to FIG. 5(e)). FIG. 5(c) shows the system current I.sub.U.sup.-. It rises to the overcurrent region by the grounding accident, but it falls to zero by the GB operation mentioned above after that.
FIG. 5(d) shows the the operation waveform of the reactive current feedback signal I.sub.Q.sup.-. Since the protecting circuit 12 operates at the occurrence of the grounding accident to generate the GB (gate block) signal of the "L" potential level in place of the DB (de-block) signal of the "H" potential level generated up to that time and the reactive current reference zero controlling circuit 20 controls the reactive current reference signal I.sub.Q * to be zero, the operation waveform corresponds to this operation.
FIG. 5(e) shows the waveform of the GB/DB signal 12a generated by the protecting circuit 12. The signal 12a becomes the GB signal of the "L" potential level at the same time as the occurrence of the accident to execute the gate block of the single-phase inverter 4 and to control the the reactive current reference signal I.sub.Q * to be zero. After the prescribed time T.sub.1 passed in keeping this state, the signal 12a releases the gate block and returns to the original DB signal of the "H" potential level with keeping the reactive current reference signal I.sub.Q * zero control.
Then, after the prescribed time T2 passed in keeping this state, the signal 12a releases the reactive current reference signal I.sub.Q * zero control and returns to the ordinary operation state.
Now, the reason why the gate block (GB) is released before the release of the reactive current reference signal I.sub.Q * zero control is that the switching devices of the single-phase inverter 4 require a certain time to shift from the GB state to the DB state. The reactive current feedback signal I.sub.Q- responds to the release of the reactive current reference signal I.sub.Q * zero control immediately and rises (FIG. 5(d)) by employing the controlling sequence mentioned above.
Since the prior art SVG is composed as mentioned above, the reactive power generator executes the protection operation using the gate block immediately after the occurrence of the grounding accident. However, it has problems as follows, since it employs the system to resume its operation on the assumption that the time period from the occurrence of an accident to the recovery is the prescribed time (T.sub.1 +T.sub.2).
That is, when the period of the grounding accident duration is shorter than the time (T.sub.1 +T.sub.2), the resumption of the operation results in being retarded unnecessarily and the shake-stabilizing effect of the system by auto-returning becomes little.
Conversely, when the period of the grounding accident duration is longer than the time (T.sub.1 +T.sub.2), the resumption of the operation results in being resumed during the grounding accident. In this case, overcurrent is easy to flow as the system voltage is unsettled. Then it may be needed to execute the protecting operation again, and there is some possibility of repeating this operation stop and being not able to return to the normal operation if circumstances require.